feat(zipper): add function to map on cursor focus with default

2 files changed, 9 insertions, 0 deletions

diff --git a/lib/zipper.ml b/lib/zipper.ml
index 30be8eb..9313518 100644
--- a/lib/zipper.ml
+++ b/lib/zipper.ml
@@ -107,6 +107,11 @@ let map_focus f z =
   | None -> z
   | Some (h, t) -> { z with after = Sequence.shift_right t (f h) }
 
+let map_focus_or ~default f z =
+  match Sequence.next z.after with
+  | None -> { z with after = Sequence.singleton default }
+  | Some (h, t) -> { z with after = Sequence.shift_right t (f h) }
+
 let map f z =
   { z with before = Sequence.map ~f z.before; after = Sequence.map ~f z.after }
 
diff --git a/lib/zipper.mli b/lib/zipper.mli
index beeb181..e25d57b 100644
--- a/lib/zipper.mli
+++ b/lib/zipper.mli
@@ -198,6 +198,10 @@ val map_after : ('a -> 'a) -> 'a zipper -> 'a zipper
 val map_focus : ('a -> 'a) -> 'a zipper -> 'a zipper
 (** Map a function over the element focused by the cursor, if any. *)
 
+val map_focus_or : default:'a -> ('a -> 'a) -> 'a zipper -> 'a zipper
+(** Map a function over the element focused by the cursor. Push
+    [default] if no element is focused. *)
+
 val map : ('a -> 'a) -> 'a zipper -> 'a zipper
 (** Map a function over all elements of a zipper. *)